How strong is the evidence for Multivitamins?

The most rigorous recent evidence comes from the COSMOS trial, a large randomized study that found daily multivitamin supplementation modestly improved memory and cognitive function in older adults over multiple years of follow-up, corroborating earlier smaller randomized trials including in the Physicians' Health Study II. That evidence is currently rated moderate (60/100), built on 10 published studies, including 5 randomized controlled trials and 4 meta-analyses -- a reasonable base, but not yet enough for full certainty.